Large scale immersion bath for isothermal testing of lithium-ion cells

Graphical abstract

Immersion cooling Lithium-ion Battery Oil Temperature control Thermal management a b s t r a c t Testing of lithium-ion batteries depends greatly on accurate temperature control in order to generate reliable experimental data. Reliable data is essential to parameterise and validate battery models, which are essential to speed up and reduce the cost of battery pack design for multiple applications. There are many methods to control the temperature of cells during testing, such as forced air convection, liquid cooling or conduction cooling using cooling plates. Depending on the size and number of cells, conduction cooling can be a complex and costly option. Although easier to implement, forced air cooling is not very effective and can introduce significant errors if used for battery model parametrisation. Existing commercially available immersion baths are not cost effective ($£3320) and are usually too small to hold even one large pouch cell. Here, we describe an affordable but effective cooling method using immersion cooling. This bath is designed to house eight large lithium-ion pouch cells (300 mm Â 350 mm), each immersed in a base oil cooling fluid (150L total volume). The total cost of this setup is only £1670. The rig is constructed using a heater, chilling unit, and a series of pumps. This immersion bath can maintain a temperature within 0.5°C of the desired set point, it is operational within the temperature range 5-55°C and has been validated at a temperature range of 25

Hardware description
We had five requirements for our immersion bath, as listed below: The capability to both chill and heat the immersion fluid; Precision temperature control of the immersion fluid to within ±0.5°C; Simple and robust temperature control system; Sized to accommodate eight large lithium-ion pouch cells (>50 Ah); Low cost.
Immersion baths available on the market target single cell testing applications or are borrowed from other research fields. Bath sizes are typically an order of magnitude smaller than our requirement. This immersion bath design meets the five requirements at a fraction of the cost of the smaller immersion baths in the market and is based on components that can easily be sourced.
The immersion bath is made of six main components ( Fig. 1): tank, test component holding frames, the heater, the chiller, the bulk flow pump and the heater/chiller mixing pump. In addition, the piping topology is arranged to ensure thorough mixing throughout the volume of the immersion bath, and thus to maintain a uniform temperature.
The heater and chiller work alongside each other to achieve the required temperature without a complex control system. The heater is set to the desired temperature, which it can achieve with an accuracy of ±0.1°C [25]. Heat is added to the immersion bath at a higher rate than the chiller can remove heat. The chiller is used for temperature stabilisation, providing a near constant rate of heat removal to reduce the overshoots achieved by the heater. This is achieved by setting the chiller to a temperature below that of the heater. The immersion bath will therefore settle at the temperature selected on the heater (see Table 3 for further details).

Design files
The design files for the immersion bath can be found in Table 1.

Bill of materials
The bill of materials for making of one unit of the immersion bath is shown in Table 2.

How temperature control is achieved
Cells and other components are arranged in the bath in two different regions (Fig. 2). The first region houses the heater, chiller and pumps with the second region housing the cells. However, no physical barrier separates these regions. It is important to ensure that the heater switches off once a uniform temperature is reached across both regions of the bath. This can only happen if the temperature probe in the heater is able to read an accurate temperature representative of both regions of bath. This has been achieved by having two pumps. The primary pump ensures uniform mixing of the oil between the two main regions of the bath. The secondary pump is near the chilling and heating elements to ensure good mixing locally. The primary pump was used to achieve good mixing between the two regions of the bath. By carefully laying out the piping arrangement, oil was pumped to each location where cells were mounted to ensure uniform flow across every cell and hence uniform temperature. The secondary pump was used to allow for good mixing in the proximity of the heating and chilling elements in the oil. This was important for two reasons. The first was that local heating or chilling does not directly affect the temperature probe in the heater. The second reason was that a uniform mixing of the oil was achieved locally at the intake of the primary pump.
Temperature stability was one of the key requirements in the design of this apparatus and this was achieved by allowing the heater and chiller to work alongside each other. This was because the chiller counterbalances the unwanted temperature rise if an internal or external heat source other than the heater was to inject heat into the bath. Two main internal heat sources are present, the heat generated by the cells during testing and the heat from the pumps during operation. This is in addition to the variability of the external environmental temperature.
In use, the heater switches off automatically when the set temperature is reached. However, significant heat is produced from the pump and dissipated into the bath even when the heater is idle. The chiller, set to cool the bath at a constant rate throughout testing, counterbalances this heating effect. During periods where the chiller is removing excessive heat, the heater is automatically triggered into operation. In this manner, the bath temperature is maintained by the heater's intermittent operation, without needing a combined control system for the heater and chiller together.

Build instructions
The bath container, its frame and insulation Fig. 3 on the edge of the tank. 2. Build a rectangular frame using aluminum extrusions and brackets. This frame will be referred to as the main frame in the later stages of the assembly. 3. It is important to add the insulation to all sides of the tank in the early stages of the assembly to ease the process. This is particularly important for the bottom of the tank. The insulation can be cut to the dimensions required, then attached to the sides of the tank by applying adhesive. 4. Attach rectangular frame to the top of the tank using mending plates (Fig. 4).

Pump and piping
1-Take measurements across the length of the tank from the primary pump for the desired exit locations of the oil at the different cell locations. Then cut pipe to dimension. 2-Pipe elbows, t-connectors, and reducers should then be connected to the desired locations measured in the previous step. Dry-fit (without applying Poly-Vinyl Chloride (PVC) pipe cement) at this stage to allow for changes that may need to be made. 3-Assemble piping arrangement and primary pump in the tank to ensure it is in the correct location and make any necessary adjustments (Fig. 5). 4-Apply PVC pipe cement to secure the pipe connections. 5-Place secondary pump in the desired location (see Fig. 2).
Heater and chiller 1-The aluminum extrusion bar to which the heater will be attached to, should be attached to the main frame using angle brackets. 2-The heater should then be attached to the bar. 3-The chiller should be attached to a copper coil using flexible pipes. Care should be taken such that the water in the loop of the chiller does not mix with the oil. 4-The copper coil can then be hung from the edge of the tank in its allocated location.
Cells 1-Each cell is mounted on a jig -provided by the cell supplier (Fig. 6). 2-Two jigs should be attached to an aluminum extrusion bar prior to the bar being attached to the main rectangular frame. Fig. 5. Primary pump along with its piping assembly. This is the view in which it was placed withing the length of the tank.

Fill bath
At this stage, the bath should be filled with oil or other desired fluid depending on the application. In applications where electrical conductivity is not an issue, de-ionised water could be used, along with inhibitor and biocide to prevent mould growth.
Cover 1-The cover was laser cut into two sections to allow for easier assembly. Profiles were laser cut on the cover to allow for the cables from the cells to pass through. Depending on the application that this immersion bath will be used for, the lid can be further split into smaller sections to allow an easier assembly process. 2-A layer of insulation should then be placed on top of the cover to minimize heat exchange between the fluid within the bath and the environment.

Operation instructions
1-Turn pumps on 2-Turn chiller on 3-Turn heater on 4-Use Table 3 as a guide for setting the temperatures on the chiller and heater depending on a required bath temperature

Validation
It was essential that the immersion bath design facilitated effective and uniform cooling across the jigs that hold the cells. For this reason, as well as ensuring thorough mixing of the immersion fluid, it was important to achieve consistent rates of heat transfer from both surfaces of every jig. This would prevent the build-up of a temperature difference from one surface of a jig to the other.  Two sets of tests were performed to confirm that the requirements of thorough bulk coolant mixing, and uniform jig cooling were both met. Preliminary tests were performed with water, and actual validation was carried out using oil. The advantage of performing preliminary tests with water first was that it was easier to drain and make any necessary modifications than it would be once the tank is filled with oil.
During both the water tests and the oil tests, the temperature of the cells (or jigs that hold the cells) was measured using K-type thermocouples mounted on the surface at the centre of each jig. Pico Technologies TC-08 units were used for temperature logging. The location where the cells were mounted in shown in Fig. 7. Please note, the accuracy of K-type thermocouples is typically ±1.5°C over the range -40 to +1000°C.

List of symbols used in the validation section
The symbols used throughout the validation section are defined in Table 4.

Preliminary water tests
A first batch of tests is performed using water, to ensure the rig is working satisfactorily before adding oil because adding oil to the rig too early would complicate any subsequent modifications or adjustments. The purpose of these tests is twofold: first, to test the overall behaviour of the immersion bath including the heater, chiller, pump and thermocouples; and second, to test the effectiveness by monitoring any temperature imbalances between the compression jig surfaces. Five water tests were conducted without the presence of the cells to avoid short circuiting the cells.
The first and second test looked at investigating temperature homogeneity between the seven cells, mounted into the seven jigs in the immersion tank ( Table 5). The first test heated the bath to 25°C from an initial temperature of 16°C (Fig. 8). The second test heated the bath to 35°C from an initial temperature of 25°C. The difference in average cell temperature was found to be small (<0.5°C). The temperature gradient across each jig was also very low (<0.1°C), therefore, it is safe to assume the immersion fluid is thoroughly mixed throughout testing. In addition, during the heating process, the temperature for each jig followed the same path and reached the setpoint at a very similar time, therefore it is assumed that the heating/cooling rate at each surface of the compression jig is equal. Fig. 7. Cell/jig locations are numbered from one to seven in order of from left to right. The front of the tank is also labelled, as it is referenced in the thermocouple location, temperature data. Two thermocouples spaced 20 cm apart in depth, were placed at each of the two locations in the tank shown with an X and also a thermocouple was placed at the centre of every jig. Table 4 List of symbols used.

T initial
Temperature of the fluid at the start of the experiment T final Temperature of the fluid at the end of the experiment oT/ot Rate of temperature change DT between jigs Temperature difference between jigs DT jig sides Temperature difference between front and back side of each jig T Instantaneous temperature of the cooling fluid t Duration the temperature was kept constant DE Thermal energy added to the tank during thermal perturbation test t mixing Time taken for the fluid to return to an isothermal condition after experiencing a forced thermal perturbation DT 15 s Maximum temperature difference between jigs after 15 s of a forced thermal perturbation DT 120 s Maximum temperature difference between the jigs after 120 s of a forced thermal perturbation DT between tank top and bottom Temperature difference between top and bottom of the tank DT tank front and back Temperature difference between front and back of the tank DT cells Temperature difference between different cells The third and fourth test looked at temperature stability (Table 6). Both parts looked at temperature stabilisation over ten hours, the first part was done at 25°C (see Fig. 8 left) and the second part at 35°C. The results showed that the temperature control system can maintain a very consistent bath temperature, with fluctuations of < 0.1°C.
The fifth test analysed the response of the system to a thermal perturbation of 250 kJ, added at the site of jig location one over a period of five seconds in the form of boiling water ( Table 7). The equilibrium time after which the water had fully mixed was determined, as well as the maximum temperature difference between jigs at 15 s and 120 s after the addition of hot water. The results showed that after 132 s the fluid had returned to thermal equilibrium after being exposed to a sudden thermal imbalance (Fig. 9). The time for the temperature gradient across each cell jig to drop below 0.5°C was just 15 s. This demonstrates that the recirculation system is very effective at mixing the immersion fluid.
This apparatus was designed to be used for oil. All tests so far in this section were performed with water as the immersion fluid. However, oil and water have different fluid properties. It is well established that base oil and water have different thermal properties and therefore are expected to perform differently as immersion fluids for lithium-ion cells. Fig. 8. Temperature from the first test whilst bath was heated to 25°C (left) and maximum difference between the different cells in the initial stage of heating up to a temperature of 25°C (right).

Oil tests with cells at rest
The purpose of these experiments was to assess the overall functioning of the immersion bath and to measure the time taken to reach a stable temperature when using oil. The temperature of the bath was both increased and decreased so that the time taken in both scenarios could be measured. Temperature across different locations in the bath and on the surface of two of the cells in resting were recorded. The results are summarised in Table 8 and Table 9. The temperature measurements from both different experiments are shown in Fig. 10 and Fig. 11. Table 8 Heating and cooling assessment up to 35°C using oil. The cells were placed in locations 5 and 7. The thermocouples in the tank are located in opposite corners front and back of the tank (Fig. 7) with two thermocouples in each location, one on the top and one at the bottom of the tank.  Table 9 Heating and cooling assessment up to 45°C using oil. The cells were placed in locations 5 and 7. The thermocouples in the tank are in opposite corners front and back of the tank (Fig. 7) with two thermocouples in each location, one on the top and one at the bottom of the tank.   Oil tests while cells undergoing constant current cycling An experiment was also carried out while the cell was being electrochemically charged and discharged at constant current (CC). The aim of this experiment was to observe any variation in tank temperature as a result of the cells heating up during electrochemical cycling. One of the cells was cycled at a moderate C-rate 1 (1C -taking one hour to charge the cell) and mounted in jig 5. The other cell was cycled at a relatively high rate (3C -taking a third of an hour to charge the cell) and mounted in jig 7.
This experiment was carried out at a tank temperature of 25°C (Fig. 12) and at 45°C (Fig. 13). The results showed that despite the cells surface temperature rising, the impact on overall tank temperature was almost negligible.

Conclusions
We have presented a low-cost, robust temperature control system for immersion cooling of large lithium-ion pouch cells, capable of controlling the temperature of the bath to within 0.5°C. Despite its large size (150 L), the design presents efficient fluid mixing, leading to homogeneous temperatures across the bath. Besides the thermal management of large lithium-ion batteries, this design is also suitable for the thermal management of supercapacitors, lithium-hybrid capacitors and any other type of large batteries, electrical components, or even non-electrical components. The oil used in the tank may be replaced with other fluids depending on the application and/or budget constraints.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Dr Yatish Patel is Advanced Research Fellow at Imperial College London in the Electrochemical Science & Engineering Group. Having trained as a physicist before moving to engineering, His research is based around developing engineering solutions centred around scientific understanding of lithium-ion batteries. His research portfolio focuses on understanding degradation mechanisms, failure modes and the limits of operation of batteries and the impact these have on system design and control.
Prof Gregory Offer leads the Electrochemical Science & Engineering Group in the Mechanical Engineering Department. Greg's research is at the interface between the science and engineering of electrochemical devices. Having trained as an electrochemist before moving to engineering, his research portfolio focuses on understanding the limits of operation, degradation mechanisms and failure modes of batteries, supercapacitors and fuel cells in real world applications, and the impacts and consequences on system design, integration and control.